Musicians who play electronic instruments, such as electric guitars, use various electronic circuits to change or augment the sound. For example, a guitarist can add distortion to the sound to create an aggressive sound which has become very popular in rock music. By housing the distortion circuitry in a foot-operated pedal, the effect can be turned off and on during a performance leaving the hands free to play the instrument. The distortion foot pedal was among the first and simplest of all guitar effects processing devices and was an analog effect inserted between the guitar and the amplifier.
Subsequently, a variety of both analog and digital single effects have become available to the musician either as foot-operated pedals or rack-mounted signal processing devices. Typically, an electric guitarist will have several foot pedals, each turning on and off a different circuit creating a variety of sounds. Examplary effects provided by such devices include delays (echoes), tremolo's (amplitude vibrato), “wah-wah” (just like it sounds), reverb, and others. Such effects, pedals, and rack-mounted processors add variety in tonal possibilities and are used by many guitarists to provide a large variety of sounds.